Solid-liquid mixtures, which are generated in chemical production processes, waste recycling or energy generation from wastes, disposal of wastes or in food technology and processing technology, are usually dewatered and dried in two separate plants, e.g., by mechanical dewatering with centrifuges, belt presses or filter presses in a first step and chemical drying in rotary drum, disk or paddle trough mixers in a subsequent, second step. The multistep plants require a complicated device and are cost-intensive. Since the multistep plants are usually operated continuously, drying must be carried out in the shortest contact times possible. In addition, great temperature differences of 100° C. to 500° C. are needed between the material being dried and the heating surfaces or dying means. The temperature range must be between 150° C. and 600° C. The use of thermal energy at this high temperature level, which otherwise remains reserved for power generation, for use for evaporating water alone in drying plants, is not meaningful economically and ecologically.
A device for dewatering and drying solid particles dispersed in liquids is known from EP 0 263 197 B1, in which the mechanical dewatering and subsequently the thermal drying are carried out in a single device, avoiding complicated, multistep plants and apparatus. The dewatering and drying is carried out intermittently in batch operation. Contrary to the continuously operated drying methods, the contact times are markedly longer in intermittent batch operation. The drying can therefore be carried out with small temperature differences of 10° C. to 90° C. between the heating surface and the material being dried at a temperature level of 20° C. to 130° C. However, the slow drying of the material from the outside and inside in the form of a drying front, which migrates from the heating surface into the interior of the material being dried, is a drawback of this method called simple vacuum drying. An extremely dry layer, which acts as an insulating wall, is formed in the material being dried at the heating surface. The heat transport into the interior of the material being dried is greatly hindered hereby and this insulating wall progressively becomes thicker in parallel to the heating surface as drying progresses. Very long drying times are needed for reaching the goal of drying as a consequence of the formation of this layer.
EP 0 759 318 B1 discloses a method for dewatering and drying solid-liquid mixtures, in which the drying operation is effectively accelerated in a single device by oscillating between two pressure levels or two temperature levels. The heat transport from a diaphragm heated with a heat carrier medium to the evaporation front in the interior of the material being dried and the vapor transport from the inside to the outside are markedly improved by the introduction of rinsing gas compared to the solution according to EP 0 263 197 B1. The rinsing gas reduces the buildup of an insulating wall in parallel to the heating surface. Thus, the heat transport persists via liquid bridges into the interior of the material being dried and the drying times become shorter during progressing drying as well. However, the drying times are, on the whole, still too long. The drying method according to EP 0 759 318 B1 is therefore not commercially applicable as yet compared to the continuously operated two-stage drying plants. Competitiveness can be achieved with further improvements only.
The heat transfer from the heat carrier medium to the material being dried is composed of the heat fluxes in the individual layers of material located one after the other. Since materials consisting of polypropylene, EPDM or other plastics are usually used for the diaphragms, the heat transfer from the heat flux through the material of the diaphragm selected is limited, and the plastics used have a low thermal conductivity and low temperature stability.
The heat transport can be improved by using heating surfaces made of metal. Thus, a welded chamber hot plate made of special steel with typical meandering guiding of the heating water, with which relatively short drying times can be reached, is known from the patent specification according to CH 685 805 A5. However, the welded chamber hot plates made of special steel have the drawback that the costs of the starting material and also the manufacturing costs are very high. In addition, the durability of the weld scams is greatly reduced under the usual drying conditions (high temperatures/corrosive liquids). However, welded chamber hot plates made of metal have not, on the whole, proved successful.